KR102576202B1 - Boil Off Gas Treatment System And Method For Ship - Google Patents

Abstract

A system and method for treating boil-off gas of a ship are disclosed. The ship boil-off gas treatment system of the present invention includes a multi-stage compressor that receives boil-off gas generated from liquefied gas stored in a storage tank on board and compresses it to the fuel supply pressure of the propulsion engine; A low-pressure fuel supply line that supplies boil-off gas compressed through some stages of the multi-stage compressor to a power generation engine with a lower fuel supply pressure than the propulsion engine; A gas analyzer provided in the low-pressure fuel supply line to analyze the nitrogen content of boil-off gas supplied to the power generation engine; A re-liquefaction line that cools and re-liquefies the boil-off gas compressed in the multi-stage compressor and returns it to the storage tank; A gas-liquid separator provided in the re-liquefaction line to separate gas-liquid from the re-liquefied gas; A gas discharge line discharging gas from the upper part of the gas-liquid separator; and a gas discharge valve provided in the gas discharge line, wherein the gas discharge valve is automatically opened and closed according to the nitrogen content of the boil-off gas analyzed by the gas analyzer.

Description

Boil Off Gas Treatment System And Method For Ship {Boil Off Gas Treatment System And Method For Ship}

The present invention relates to a system and method for processing boil-off gas of a ship, and more specifically, in a ship equipped with a propulsion engine and a power generation engine supplied with lower-pressure fuel than the propulsion engine, boil-off gas is compressed and supplied as fuel for the engine, and then re-processed. A ship's boil-off gas treatment system and method that analyzes the nitrogen content of the boil-off gas supplied to the power generation engine while liquefying it and discharges the flash gas separated from the gas-liquid separator according to the nitrogen content of the analyzed boil-off gas to prevent engine tripping. It's about.

Recently, the consumption of liquefied gas such as liquefied natural gas (LNG) is rapidly increasing worldwide. Liquefied gas, which is made by liquefying gas at low temperature, has a much smaller volume than gas, so it has the advantage of increasing storage and transportation efficiency. In addition, liquefied gas, including liquefied natural gas, can remove or reduce air pollutants during the liquefaction process, so it can be viewed as an eco-friendly fuel with low emissions of air pollutants during combustion.

Liquefied natural gas is a colorless and transparent liquid obtained by liquefying natural gas containing methane as a main component by cooling it to about -163°C, and has a volume of about 1/600 of that of natural gas. Therefore, when natural gas is liquefied and transported, it can be transported very efficiently.

However, the liquefaction temperature of natural gas is extremely low at -162°C at normal pressure, so liquefied natural gas is sensitive to temperature changes and easily evaporates. For this reason, the storage tank that stores liquefied natural gas is insulated, but external heat is continuously transferred to the storage tank, so during the transportation of liquefied natural gas, liquefied natural gas is continuously naturally vaporized within the storage tank, producing boil-off gas (boil). -Off Gas, BOG) occurs.

Evaporation gas is a type of loss and is an important issue in transportation efficiency. In addition, if evaporation gas accumulates in the storage tank, the pressure inside the tank may increase excessively, and in severe cases, there is a risk of tank damage. Therefore, various methods are being studied to treat boil-off gas generated within the storage tank. Recently, for the treatment of boil-off gas, a method of re-liquefying the boil-off gas and returning it to the storage tank, using the boil-off gas as fuel for ship engines, etc. Methods such as using it as an energy source for consumers are being used.

Methods for re-liquefying the boil-off gas include a method of re-liquefying the boil-off gas by heat-exchanging it with a refrigerant using a refrigeration cycle using a separate refrigerant, and a method of re-liquefying the boil-off gas itself as a refrigerant without a separate refrigerant. There is.

Meanwhile, among engines generally used in ships, engines that can use natural gas as fuel include gas fuel engines such as DFDE, X-DF engines, and ME-GI engines.

DFDE consists of a 4-stroke cycle and adopts the Otto Cycle, which injects natural gas with a relatively low pressure of about 5.5 barg into the combustion air inlet and compresses it as the piston rises.

The X-DF engine consists of a two-stroke engine, uses natural gas of about 15 barg as fuel, and adopts the Otto cycle.

The ME-GI engine consists of two strokes and adopts the diesel cycle, which injects high-pressure natural gas around 300 barg directly into the combustion chamber near the top dead center of the piston.

The present applicant is a method of re-liquefying the boil-off gas by using the boil-off gas itself as a refrigerant without a separate refrigerant. The boil-off gas compressed by the compressor is cooled by heat exchange with the boil-off gas before being compressed by the compressor, and then cooled by a J-T valve, etc. A method of re-liquefying part of the boil-off gas by expanding it has been invented, and this system is called PRS (Partial Re-liquefaction System).

When there is a large amount of boil-off gas that needs to be re-liquefied, such as when the amount of liquefied gas inside the storage tank is large and a large amount of boil-off gas is generated, or when the ship is anchored or operates at a low speed and the amount of boil-off gas used in the engine is small. , PRS alone may not be able to satisfy the required amount of re-liquefaction, so the present applicant invented a technology to improve PRS so that more boil-off gas can be re-liquefied.

As an improved technology of PRS, the system that allows additional cooling of boil-off gas through a refrigerant cycle using the boil-off gas itself as a refrigerant is called MRS (Methane Refrigeration System).

A separate refrigerant, such as a mixed refrigerant or nitrogen, may be used to cool the boil-off gas to be reliquefied.

Meanwhile, in ships equipped with engines that can use boil-off gas as fuel, a compressor can be used to re-liquefy the boil-off gas and supply fuel to the engine.

In Figure 1, in a ship equipped with an engine (ME, GE) that receives boil-off gas generated from LNG as fuel, high-pressure compressed boil-off gas is supplied as engine fuel through fuel supply compressors (10A, 10B), and the boil-off gas is supplied as fuel. A boil-off gas treatment system is schematically shown in which the unsupplied compressed gas is cooled by boil-off gas cold heat in a heat exchanger (20), then decompressed (30), gas-liquid separated (40), and returned to a storage tank.

When a propulsion engine (ME) and a power generation engine (GE) with a lower fuel supply pressure are provided, the fuel supply compressor is provided as a multi-stage compressor (10A, 10B), and the boil-off gas compressed through some stages of the multi-stage compressor is supplied to the power generation engine. It is supplied to (GE). The boil-off gas compressed through all of the multi-stage compressors is supplied as fuel to the propulsion engine (ME), and the boil-off gas not supplied as fuel is cooled in the heat exchanger (20), decompressed through the J-T valve (30), and then sent to the gas-liquid separator. The reliquefied gas is recovered to the storage tank (T) through (40), and the separated flash gas is sent to the uncompressed boil-off gas flow in front of the heat exchanger (20) through the valve (50) when the gas-liquid separator reaches a certain pressure. .

LNG may contain nitrogen. Nitrogen (-196°C), which has a lower boiling point than methane, evaporates before methane, so the nitrogen content in the boil-off gas generated in the storage tank is significant, and even if it goes through the reliquefaction process, nitrogen with a low boiling point remains. It is not re-liquefied and remains in a gaseous state.

However, when boil-off gas with high nitrogen content is supplied as engine fuel, the engine, especially the power generation engine, often trips, and engine manufacturers require certain conditions, such as Gas Quality N ₂ mole 15% or less, for the gas supplied as fuel. I demand it. However, as the re-liquefaction process continues, the nitrogen content of the boil-off gas introduced into the compressor increases and exceeds these conditions frequently, which can cause frequent trips of the engine and reduce the re-liquefaction rate.

The present invention is intended to solve this problem and proposes a method for preventing engine tripping and stably operating the reliquefaction system by controlling the nitrogen content in evaporation gas.

According to one aspect of the present invention for solving the above-described problem, a multi-stage compressor that receives boil-off gas generated from liquefied gas stored in a storage tank on board and compresses it to the fuel supply pressure of the propulsion engine;

A low-pressure fuel supply line that supplies boil-off gas compressed through some stages of the multi-stage compressor to a power generation engine with a lower fuel supply pressure than the propulsion engine;

A gas analyzer provided in the low-pressure fuel supply line to analyze the nitrogen content of boil-off gas supplied to the power generation engine;

A re-liquefaction line that cools and re-liquefies the boil-off gas compressed in the multi-stage compressor and returns it to the storage tank;

A gas-liquid separator provided in the re-liquefaction line to separate gas-liquid from the re-liquefied gas;

A gas discharge line discharging gas from the upper part of the gas-liquid separator; and

Including a gas discharge valve provided in the gas discharge line,

A ship boil-off gas treatment system is provided, wherein the gas discharge valve is automatically opened and closed according to the nitrogen content of the boil-off gas analyzed by the gas analyzer.

Preferably, a heat exchanger is provided in the reliquefaction line and cools the boil-off gas compressed in the multi-stage compressor by heat exchange with uncompressed boil-off gas to be supplied from the storage tank to the multi-stage compressor; and a decompression device provided in the re-liquefaction line to depressurize the boil-off gas cooled in the heat exchanger and supply it to the gas-liquid separator.

Preferably, a flash gas line directs the gas separated in the gas-liquid separator to an uncompressed boil-off gas flow introduced into the heat exchanger; and a pressure control valve provided in the flash gas line, wherein the pressure control valve is linked to the opening and closing of the gas discharge valve, but may operate opposite to the opening and closing of the gas discharge valve.

The gas discharge valve is opened when the nitrogen content of the boil-off gas analyzed by the gas analyzer is 15 mol % or more, and when the gas discharge valve is opened, the pressure control valve can be locked.

Preferably, the gas discharged from the gas discharge line can be transferred to a dedicated gas dome provided on board the ship.

According to another aspect of the present invention, in a ship equipped with a propulsion engine and a power generation engine supplied with lower pressure fuel than the propulsion engine,

The boil-off gas generated from the liquefied gas stored in the storage tank is compressed with a multi-stage compressor that compresses it up to the fuel supply pressure of the propulsion engine, cooled through a heat exchanger, re-liquefied, and then gas-liquid separated with a gas-liquid separator, and the re-liquefied gas is transferred to the storage tank. Recover,

In the heat exchanger, the boil-off gas compressed in the multi-stage compressor is cooled by heat exchange with the uncompressed boil-off gas to be supplied to the multi-stage compressor, and the flash gas separated in the gas-liquid separator is supplied to the uncompressed boil-off gas flow,

While supplying the compressed boil-off gas through some stages of the multi-stage compressor to the power generation engine,

The nitrogen content of the boil-off gas supplied to the power generation engine is analyzed, and if the nitrogen content of the analyzed boil-off gas is above a certain value, the flash gas separated in the gas-liquid separator is discharged without sending it to the uncompressed boil-off gas flow. A method for treating boil-off gas from ships is provided.

In the present invention, the boil-off gas generated in the storage tank is supplied as engine fuel on board the ship or re-liquefied and processed, the nitrogen content of the boil-off gas supplied to the power generation engine is analyzed, and the analyzed boil-off gas is separated in a gas-liquid separator according to the nitrogen content. The flash gas is not sent to the front of the heat exchanger but is discharged through the gas discharge line.

In this way, by controlling the nitrogen content in the fuel supply and re-liquefied boil-off gas, high-quality fuel gas can be supplied, engine trip can be prevented, and the re-liquefaction system can be operated stably.

Figure 1 schematically shows an example of a conventional boil-off gas treatment system.
Figure 2 schematically shows a ship's boil-off gas treatment system according to an embodiment of the present invention.

In order to fully understand the operational advantages of the present invention and the objectives achieved by practicing the present invention, reference should be made to the accompanying drawings illustrating preferred embodiments of the present invention and the contents described in the accompanying drawings.

Hereinafter, the structure and operation of a preferred embodiment of the present invention will be described in detail with reference to the attached drawings. Here, in adding reference numerals to components in each drawing, it should be noted that identical components are indicated with the same reference numerals as much as possible, even if they are shown in different drawings.

Hereinafter, the ship in the present invention is any type installed with an engine that can use liquefied gas and boil-off gas generated from the liquefied gas as fuel for propulsion or power generation engines, or uses liquefied gas or boil-off gas as fuel for the ship's engines. of ships, including ships with self-propelled capabilities such as LNG carriers, liquid hydrogen carriers, and LNG RVs (Regasification Vessels), as well as LNG FPSOs (Floating Production Storage Offloading) and LNG FSRUs (Floating Storage Regasification Units). ) may also include offshore structures that do not have propulsion capabilities but are floating in the sea.

In addition, the liquefied gas in the present invention may include all types of liquefied gas that can be transported by liquefying gas at low temperature, generate boil-off gas in a stored state, and can be used as fuel for engines, etc. These liquefied gases are, for example, liquefied petrochemicals such as LNG (Liquefied Natural Gas), LEG (Liquefied Ethane Gas), LPG (Liquefied Petroleum Gas), Liquefied Ethylene Gas, and Liquefied Propylene Gas. It could be gas. However, in the examples described later, the application of LNG, one of the representative liquefied gases, will be described as an example.

Meanwhile, the fluid flowing through each line of the present embodiments may be in any one of a liquid state, a gas-liquid mixture state, a gas state, and a supercritical fluid state, depending on the operating conditions of the system.

Figure 2 schematically shows a ship's boil-off gas treatment system according to an embodiment of the present invention.

As shown in Figure 2, the system of this embodiment is for processing boil-off gas generated in a storage tank provided on a ship and storing liquefied gas, and includes a compressor ( 100A, 100B), includes a propulsion engine (ME) and a power generation engine (GE) that are supplied as fuel by boil-off gas compressed in a compressor.

The compressors (100A, 100B) compress the boil-off gas transported from the storage tank along the gas supply line (GL), and are used for propulsion when the ship is equipped with a propulsion engine (ME) and a power generation engine (GE) with a lower fuel supply pressure. It can be equipped with a multi-stage compressor that compresses the evaporative gas up to the fuel supply pressure of the engine. For example, a multi-stage compressor can compress to around 15 barg if an X-DF engine is provided as the propulsion engine, and around 300 barg if a ME-GI engine is provided.

Boil-off gas compressed through some stages of a multi-stage compressor can be supplied as fuel for a power generation engine (GE), which has a lower fuel supply pressure than the propulsion engine. For example, if a DFDG or DFGE engine is provided, boil-off gas compressed to 5 to 10 bara can be supplied to the power generation engine through some stages of a multi-stage compressor. In order to supply boil-off gas as fuel to the propulsion engine and power generation engine, a high-pressure fuel supply line (HFL) is connected from the rear end of the multi-stage compressor to the propulsion engine (ME), and low-pressure fuel is supplied from the middle stage of the multi-stage compressor to the power generation engine (GE). The line (LFL) is connected. The low-pressure fuel supply line (LFL) is equipped with a gas analyzer 500 that analyzes the nitrogen content of the boil-off gas supplied to the power generation engine. The gas analyzer may be, for example, a gas chromatograph that analyzes the composition of the gas supplied as fuel.

According to ship regulations, compressors that supply fuel to engines must be designed with redundancy in preparation for emergency situations. Redundancy design means that when one compressor cannot be used due to breakdown, maintenance, etc., the compressor that supplies fuel to the engine must be designed with redundancy in place of the other compressor. This means designing it so that it can be used. Accordingly, the compressor may be configured to include a main compressor (100A) and a redundancy compressor (100B).

Boil-off gas that is not supplied as fuel for the propulsion engine or power generation engine can be sent to the re-liquefaction line (RL) to be re-liquefied. For this purpose, a reliquefaction line (RL) is connected to the storage tank (T) at the rear of the multi-stage compressor (100A, 100B), and the boil-off gas compressed in the multi-stage compressor is reliquefied through a cooling and decompression process and returned to the storage tank.

The reliquefaction line (RL) includes a heat exchanger (200) for cooling the boil-off gas compressed in the compressor, a pressure reducing device (300) for depressurizing the boil-off gas cooled by heat exchange, and a heat exchanger and pressure reducing device to cool the boil-off gas. A gas-liquid separator 400 is provided to separate gas-liquid and supply liquefied gas to a storage tank.

In the heat exchanger, the boil-off gas compressed in the multi-stage compressor can be cooled by heat exchange with the uncompressed boil-off gas to be supplied from the storage tank to the multi-stage compressor. For this purpose, the gas supply line (GL) is connected from the storage tank (T) to the multi-stage compressors (100A, 100B) through a heat exchanger. When the reliquefaction device is not in operation or at the beginning of operation of the reliquefaction device, the boil-off gas from the storage tank can be supplied directly to the multi-stage compressor, bypassing the heat exchanger.

The boil-off gas cooled while passing through the heat exchanger may be further cooled by adiabatic expansion or isentropic expansion through reduced pressure in the pressure reducing device 300 and then separated into gas and liquid in the gas-liquid separator 400. The pressure reducing device may be provided, for example, a J-T valve or an expander.

A liquid level control valve (LCV) is provided downstream of the gas-liquid separator 400 of the re-liquefaction line (RL), and is opened and closed according to the liquid level of the gas-liquid separator to allow the re-liquefied gas separated from the gas-liquid separator to be transferred to the storage tank (T). You can.

The gas separated from the gas-liquid separator 400, flash gas, is supplied to the front of the heat exchanger 200 of the gas supply line (GL) through the flash gas line (FL) and joins the uncompressed boil-off gas flow from the storage tank. . A pressure control valve (PCV) is provided in the flash gas line.

However, nitrogen contained in LNG has a lower boiling point than methane (-196°C), so it evaporates before methane, and even after going through the re-liquefaction process, it is not re-liquefied and remains in a gaseous state. Therefore, as the re-liquefaction process continues, as flash gas with high nitrogen content continues to join the gas supply line (GL) along the flash gas line (FL), the nitrogen content of the boil-off gas introduced into the multi-stage compressor gradually increases, causing damage to power generation engines or The gas fuel supplied to the propulsion engine may not meet the gas quality required by the engine manufacturer for the gas supplied as fuel.

If evaporative gas, which has a high nitrogen content and does not meet gas quality requirements, is supplied as fuel to the engine, it can cause frequent trips in the engine, especially in power generation engines.

To prevent this, the system of this embodiment provides a gas discharge line (NL) that discharges gas from the upper part of the gas-liquid separator 400, and flash gas of the gas-liquid separator according to the nitrogen content of the boil-off gas analyzed by the gas analyzer 500. Instead of sending it to the flash gas line, it can be discharged through the gas discharge line (NL).

A gas discharge valve (NCV) is provided in the gas discharge line (NL), and the gas discharge valve (NCV) is automatically opened and closed according to the nitrogen content of the boil-off gas analyzed by the gas analyzer 500.

In addition, the pressure control valve (PCV) is linked to the opening and closing of the gas discharge valve (NCV), but operates in opposition to the opening and closing of the gas discharge valve, so that the flash gas separated from the gas-liquid separator is transported by selecting either the flash gas line or the gas discharge line. do.

The gas discharged from the gas discharge line (NL) can be transferred to a dedicated gas dome provided on board and recycled on board, and there is a safety valve (SV) at the top of the gas-liquid separator that can adjust the pressure of the gas-liquid separator in case of emergency. is prepared.

The gas discharge valve (NCV) can be set to open and close automatically according to the gas quality conditions required by the engine manufacturer of the power generation engine. For example, it is set to open when the nitrogen content of the boil-off gas analyzed by the gas analyzer is more than 15 mol %. It can be. When the gas discharge valve (NCV) is opened, the pressure control valve (PCV) is closed to send the flash gas from the gas-liquid separator to the gas discharge line (NL), and thus to the multi-stage compressor (100A, 100B) through the gas supply line (GL). Since the boil-off gas generated from the storage tank (T) is supplied, high-quality fuel gas can be supplied by lowering the nitrogen content of the boil-off gas supplied to the power generation engine and propulsion engine. When the nitrogen content of the boil-off gas analyzed by the gas analyzer (500) falls below the required level, the gas discharge valve (NCV) is automatically closed and the pressure control valve (PCV) is opened to allow the flash gas from the gas-liquid separator to flow back through the flash gas line (FL). Send it to the gas supply line (GL).

Through this system, high-quality fuel gas can be supplied to the engine to prevent abnormal phenomena such as trips, and the re-liquefaction system can be operated stably by increasing the re-liquefaction rate by controlling the nitrogen content of the gas to be re-liquefied.

It is obvious to those skilled in the art that the present invention is not limited to the above-mentioned embodiments, and that it can be implemented with various modifications or variations without departing from the technical gist of the present invention. It was done.

ME: propulsion engine
GE: power generation engine
T: storage tank
GL: Evaporative gas supply line
RL: Reliquefaction line
100A, 100B: Multi-stage compressor
200: heat exchanger
300: Pressure reducing device
400: Gas-liquid separator
500: Gas analyzer
NL: Gas discharge line
NCV: Gas discharge valve

Claims (6)

A multi-stage compressor that receives boil-off gas generated from liquefied gas stored in a storage tank on board and compresses it to the fuel supply pressure of the propulsion engine;
A low-pressure fuel supply line that supplies boil-off gas compressed through some stages of the multi-stage compressor to a power generation engine with a lower fuel supply pressure than the propulsion engine;
A gas analyzer provided in the low-pressure fuel supply line to analyze the nitrogen content of boil-off gas supplied to the power generation engine;
A re-liquefaction line that cools and re-liquefies the boil-off gas compressed in the multi-stage compressor and returns it to the storage tank;
A gas-liquid separator provided in the re-liquefaction line to separate gas-liquid from the re-liquefied gas;
A gas discharge line discharging gas from the upper part of the gas-liquid separator;
A gas discharge valve provided in the gas discharge line;
a heat exchanger provided in the reliquefaction line and cooling the boil-off gas compressed in the multi-stage compressor by heat exchange with uncompressed boil-off gas to be supplied from the storage tank to the multi-stage compressor;
a flash gas line that sends the gas separated in the gas-liquid separator to an uncompressed boil-off gas flow introduced into the heat exchanger; and
Including a pressure control valve provided in the flash gas line,
The gas discharge valve is automatically opened and closed according to the nitrogen content of the boil-off gas analyzed by the gas analyzer, with automatic opening and closing conditions set according to the gas quality conditions required by the power generation engine,
The pressure control valve is linked to the opening and closing of the gas discharge valve, but operates in the opposite direction to the opening and closing of the gas discharge valve. According to clause 1,
A decompression device provided in the re-liquefaction line and depressurizing the boil-off gas cooled in the heat exchanger and supplying it to the gas-liquid separator. delete According to clause 2,
The gas discharge valve is opened when the nitrogen content of the boil-off gas analyzed by the gas analyzer is 15 mol % or more, and when the gas discharge valve is opened, the pressure control valve is closed. A ship's boil-off gas treatment system. According to clause 4,
A ship's boil-off gas treatment system, characterized in that the gas discharged from the gas discharge line is transferred to a dedicated gas dome provided on board the ship. delete

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